Oral: Effect of encapsulation on superconducting properties of niobium thin films for qubits applications

Niobium thin films are employed in transmon qubits, serving as readout resonators, capacitor pads, and coupling lines. However, fresh metallic surface exposed to the ambient atmosphere is vulnerable to forming niobium oxides, hydroxides, hydrogen and oxygen adsorption, all detrimental to qubits performance. To address these issues, Nb films are encapsulated by thin layers of non-oxidizing metals. We investigate the superconducting properties of Nb films encapsulated by Au and PdAu, and compare them with bare Nb. The films were deposited via sputtering and molecular beam epitaxy (MBE) on sapphire substrates. The multi-modal characterization that includes magneto-optical imaging, magnetization, resistivity, and London penetration depth reveals that encapsulation significantly alters the superconducting state behavior. MBE-grown Au-capped films exhibit the most uniform flux distribution, highest superconducting transition temperature (T_c), highest residual resistivity ratio (RRR), lowest upper critical field (H_c2), and much lower vortex pinning, making them least prone to thermo-magnetic instabilities. We extract the scattering rate and superconducting gap amplitude and show that the observed variations of properties are consistent with the recent microscopic theory of anisotropic superconductivity of niobium.